Pixel driver circuit and pixel circuit having the pixel driver circuit

ABSTRACT

A pixel driver circuit for driving a light-emitting element and a pixel circuit having the pixel driver circuit are provided. The pixel driver circuit includes a data line, address lines, switch thin film transistors, feedback thin film transistors and drive thin film transistors. The pixel circuit may include an organic light emitting diode, which is driven by the pixel driver circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of U.S. patent applicationSer. No. 10/468,319 filed on Jan. 23, 2004, which is the U.S. NationalPhase of PCT/CA02/00173 having an International Filing Date of Feb. 18,2002, which claims the benefit of U.S. provisional patent applicationSer. No. 60/268,900 filed on Feb. 16, 2001, the contents of all of theforegoing applications are incorporated herein by reference in theirentirety.

FIELD OF INVENTION

The present invention relates to a display technology, and moreparticularly to a pixel driver circuit for driving a light-emittingelement and a pixel circuit having the pixel driver circuit.

BACKGROUND OF THE INVENTION

Organic light emitting diode (OLED) displays have gained significantinterest recently in display applications in view of their fasterresponse times, larger viewing angles, higher contrast, lighter weight,lower power, amenability to flexible substrates, as compared to liquidcrystal displays (LCDs). Despite the OLED's demonstrated superiorityover the LCD, there still remain several challenging issues related toencapsulation and lifetime, yield, color efficiency, and driveelectronics, all of which are receiving considerable attention.

Although passive matrix addressed OLED displays are already in themarketplace, they do not support the resolution needed in the nextgeneration displays, since high information content (HIC) formats areonly possible with the active matrix addressing scheme.

Active matrix addressing involves a layer of backplane electronics,based on thin film transistors (TFTs) fabricated using amorphous silicon(a-Si:H), polycrystalline silicon (poly-Si), or polymer technologies, toprovide the bias voltage and drive current needed in each OLED basedpixel. Here, the voltage on each pixel is lower and the currentthroughout the entire frame period is a low constant value, thusavoiding the excessive peak driving and leakage currents associated withpassive matrix addressing. This in turn increases the lifetime of theOLED.

In active matrix OLED (AMOLED) displays, it is important to ensure thatthe aperture ratio or fill factor (defined as the ratio of lightemitting display area to the total pixel area) should be high enough toensure display quality.

Conventional AMOLED displays are based on light emission through anaperture on the glass substrate where the backplane electronics isintegrated. Increasing the on-pixel density of TFT integration forstable drive current reduces the size of the aperture. The same happenswhen pixel sizes are scaled down. One solution to having an apertureratio that is invariant on scaling or on-pixel integration density is tovertically stack the OLED layer on the backplane electronics, along witha transparent top electrode as shown in FIG. 2. In FIG. 2, referencenumerals S and D denote a source and a drain, respectively. This impliesa continuous back electrode over the OLED pixel.

However, this continuous back electrode can give rise to parasiticcapacitance, whose effects become significant when the electrode runsover the switching and other TFTs. The presence of the back electrodecan induce a parasitic channel in TFTs giving rise to high leakagecurrent. The leakage current is the current that flows between sourceand drain of the TFT when the gate of the TFT is in its OFF state.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a system that obviates ormitigates at least one of the disadvantages of existing systems.

The present invention relates to a pixel driver circuit for driving alight-emitting element (e.g. OLED), and a pixel circuit having the pixeldriver circuit.

In accordance with an aspect of the present invention, there is provideda pixel driver circuit, which includes: an address line; a data line; aswitch thin film transistor, a first node of the switch transistor beingconnected to the data line and a gate of the switch transistor beingconnected to the address line; a feedback thin film transistor, a firstnode of the feedback transistor being connected to the data line and agate of the feedback transistor being connected to the address line; areference thin film transistor, a drain of the reference transistorbeing connected to a second node of the feedback transistor, a gate ofthe reference transistor being connected to a second node of the switchtransistor and a source of the reference transistor being connected to aground potential; and a drive thin film transistor, a gate of the drivetransistor being connected to the gate of the reference transistor.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit, which includes: the pixel driver circuitdescribed above; and an organic light emitting diode, the source of thedrive transistor being connected to the ground potential and the drainbeing connected to the organic light emitting diode.

In accordance with a further aspect of the present invention, there isprovided a pixel driver circuit, which includes: an address line; a dataline; a switch thin film transistor, a first node of the switchtransistor being connected to the data line and a gate of the switchtransistor being connected to the address line; a feedback thin filmtransistor, a gate of the feedback transistor being connected to theaddress line and a second node of the feedback transistor beingconnected to a ground potential; a reference thin film transistor, adrain of the reference transistor being connected to a second node ofthe switch transistor, a gate of the reference transistor beingconnected to the second node of the switch transistor and a source ofthe reference transistor being connected to a first node of the feedbacktransistor; and a drive thin film transistor, a gate of the drivetransistor being connected to the gate of the reference transistor.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit, which includes: the pixel driver circuitdescribed above; and an organic light emitting diode, the source of thedrive transistor being connected to the ground potential and the drainbeing connected to the organic light emitting diode.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit, which includes: the pixel driver circuitdescribed above; and an organic light emitting diode, the source of thedrive transistor being connected to the organic light emitting diode andthe drain being connected to a voltage supply.

In accordance with a further aspect of the present invention, there isprovided a pixel driver circuit, which includes: an address line; a dataline; a switch thin film transistor, a first node of the switchtransistor being connected to the data line and a gate of the switchtransistor being connected to the address line; a feedback thin filmtransistor, a first node of the feedback transistor being connected tothe data line and a gate of the feedback transistor being connected tothe address line; a reference thin film transistor, a drain of thereference transistor being connected to a second node of the feedbacktransistor, the gate of the reference transistor being connected to asecond node of the switch transistor and a source of the referencetransistor being connected to a ground potential; a diode-use thin filmtransistor, a drain and a gate of the diode-use transistor beingconnected to a potential, and a source of the diode-use transistor beingconnected to the second node of the feedback transistor; and a drivethin film transistor, a gate of the drive transistor being connected tothe gate of the reference transistor.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit, which includes: the pixel driver circuitdescribed above; and an organic light emitting diode, the source of thedrive transistor being connected to the ground potential and the drainbeing connected to the organic light emitting diode.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit, which includes: the pixel driver circuitdescribed above; and an organic light emitting diode, the source of thedrive transistor being connected to the organic light emitting diode,and the drain being connected to a voltage supply.

In accordance with a further aspect of the present invention, there isprovided a pixel driver circuit for driving a colour pixel of a colourdisplay, which includes: a first address line; a data line; a firstswitch thin film transistor, a first node of the first switch transistorbeing connected to the data line and a gate of the switch transistorbeing connected to the first address line; a feedback thin filmtransistor, a first node and a gate of the feedback transistor beingconnected to a second node of the first switch transistor and a secondnode of the feedback transistor being connected to a ground potential; asecond switch thin film transistor, a source of the second switchtransistor being connected to a second node of the first switchtransistor, a gate of the second switch transistor being connected to asecond address line; a first drive thin film transistor, a gate of thefirst drive transistor being connected to a drain of the second switchtransistor; a third switch thin film transistor, a source of the thirdswitch transistor being connected to the second node of the first switchtransistor, a gate of the third switch transistor being connected to athird address line; a second drive thin film transistor, a gate, of thesecond drive transistor being connected to the drain of the third switchtransistor; a fourth switch thin film transistor, a source of the fourthswitch transistor being connected to the second node of the first switchtransistor, a gate of the fourth switch transistor being connected to afourth address line; and a third drive thin film transistor, a gate ofthe third drive transistor being connected to the drain of the fourthswitch transistor.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit, which includes: the pixel driver circuitdescribed above; a first organic light emitting diode, a source of thefirst drive transistor being connected to the ground potential and adrain of the first drive transistor being connected to the first organiclight emitting diode; a second organic light emitting diode, a source ofthe second drive transistor being connected to the ground potential anda drain of the second drive transistor being connected to the secondorganic light emitting diode; and a third organic light emitting diode,a source of the third drive transistor being connected to the groundpotential and a drain of the third drive transistor being connected tothe third organic light emitting diode.

In accordance with a further aspect of the present invention, there isprovided a pixel circuit which includes: a pixel driver circuitdescribed above, a first organic light emitting diode associated withthe first drive transistor; a second organic light emitting diodeassociated with the second drive transistor; and a third organic lightemitting diode associated with the third drive transistor, the source ofthe first drive transistor being connected to the first organic lightemitting diode, and a drain of the first drive transistor beingconnected to a voltage supply.

This summary of the invention does not necessarily describe all featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1 shows variation of required pixel areas with mobility for 2-T and5-T pixel drivers;

FIG. 2 shows a conventional pixel architecture for surface emissivea-Si:H AMOLED displays;

FIG. 3 shows a cross section view of a dual-gate TFT structure;

FIG. 4 shows forward and reverse transfer characteristics of dual-gateTFT for various top gate biases;

FIG. 5 shows a panel architecture of a AMOLED display;

FIG. 6A shows a pixel circuit including a conventional 2-T pixel drivercircuit;

FIG. 6B shows input-output timing diagrams for the 2-T pixel circuit ofFIG. 6A;

FIG. 7A shows a pixel circuit including a 5-T pixel current drivercircuit for an OLED display in accordance with an embodiment of thepresent invention;

FIG. 7B shows input-output timing diagrams of the 5-T pixel circuit ofFIG. 7A;

FIG. 8 shows transient performance of the 5-T pixel current drivercircuit of FIG. 7A for three consecutive write cycles;

FIG. 9 shows input-output transfer characteristics for the 2-T pixeldriver circuit of FIG. 6A for different supply voltages;

FIG. 10 shows input-output transfer characteristics for the 5-T pixelcurrent driver circuit of FIG. 7A for different supply voltages;

FIG. 11 shows variation in OLED current as a function of the normalizedshift in threshold voltage;

FIG. 12 shows a pixel circuit including a conventional 2-T polysiliconbased pixel driver circuit having p-channel drive TFTs;

FIG. 13 shows a pixel circuit including a 4-T pixel current drivercircuit for an OLED display in accordance with a further embodiment ofthe present invention;

FIG. 14 shows a pixel circuit including a 4-T pixel current drivercircuit for an OLED display in accordance with a further embodiment ofthe present invention;

FIG. 15 shows a pixel circuit including a 4-T pixel current drivercircuit for an OLED display in accordance with a further embodiment ofthe present invention;

FIG. 16 shows a pixel circuit including a 4-T pixel current drivercircuit for an OLED display in accordance with a further embodiment ofthe present invention;

FIG. 17 shows a pixel circuit including a pixel current driver circuitfor a full color, OLED display in accordance with a further embodimentof the present invention;

FIG. 18 shows a schematic diagram of the top gate and the bottom gate ofa dual gate transistor where the top gate is electrically connected tothe bottom gate;

FIG. 19 shows a pixel circuit including a 5-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 20 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 21 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 22 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 23 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 24 shows a pixel circuit including a pixel current driver circuitfor a full color display in accordance with a further embodiment of thepresent invention;

FIG. 25 shows a pixel circuit including a 5-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 26 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 27 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 28 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention;

FIG. 29 shows a pixel circuit including a 4-T pixel current drivercircuit in accordance with a further embodiment of the presentinvention; and

FIG. 30 shows a pixel circuit including a pixel current driver circuitfor a full color display in accordance with a further embodiment of thepresent invention.

DETAILED DESCRIPTION

The following description is of a preferred embodiment.

The embodiments of the present invention are described using an OLEDdisplay. However, the embodiments of the present invention areapplicable to any other displays, such as phosphorus displays, inorganicelectroluminescent (EL), and LED displays. A pixel driver circuit inaccordance with the embodiments of the present invention includes aplurality of TFTs, which form a current mirror based pixel currentdriver for automatically compensating for the shift of threshold V_(th)of a drive TFT. The TFTs are formed in a current-programmed ΔV_(T)-compensated manner.

The pixel driver circuit is suitable for an OLED display. The OLED layermay be vertically stacked on the plurality of TFTs. The pixel drivercircuit may be provided for monochrome displays or for full colourdisplays. The OLED may be a regular (P-I-N) stack OLED or an inverted(N−1-P) stack OLED, and may be located at either the drain or source ofthe drive TFT(s)

The TFT may be an n-type TFT or a p-type TFT. The TFT may be, but notlimited to, an amorphous silicon (a-Si:H) based TFT, a polysilicon-basedTFT, a crystalline silicon based TFT, or an organic semiconductor basedTFT.

Although amorphous Si does not enjoy equivalent electronic propertiescompared to poly-Si, it adequately meets many of the drive requirementsfor small area displays such as those used in pagers, cell phones, andother mobile devices. Poly-Si TFTs have one key advantage in that theyare able to provide better pixel drive capability because of theirhigher mobility. Their higher mobility can be of the order of μ_(FE)˜100cm²/Vs. “μ_(FE)” represents field effect mobility, which is typicallyused to evaluate how well a semiconductor can conduct. “Vs” is a unitwhere V stands for volt, and s stands for second. This makes poly-Sihighly desirable for large area (e.g. laptop size) Video Graphics Array(VGA) and Super VGA (SVGA) displays. The lower mobility associated witha-Si:H TFTs (μ_(FE)˜1 cm²/Vs) is not a limiting factor since the drivetransistor in the pixel can be scaled up in area to provide the neededdrive current. The OLED drive current density is typically 10 mA/cm² at10V operation to provide a brightness of 100 cd/m², which is therequired luminance for most displays. For example, with an a-Si:H TFTmobility of 0.5 cm²/Vs and channel length of 25 μm, this drive currentrequirement translates into required pixel area of 300 μm², whichadequately meets the requirements of pixel resolution and speed for some3-inch monochrome display applications.

FIG. 1 illustrates simulation results for the variation of the requiredpixel size with device mobility calculated for two types of drivers,which will be elaborated later, a conventional voltage-programmed 2-Tpixel driver circuit (FIG. 6A) and a current-programmed,ΔV_(T)-compensated 5-T pixel driver circuit in accordance with anembodiment of the present invention (FIG. 7A)

In FIG. 1, the graph having a mark “▪” represents the pixel sizerequired by the 2T pixel driver circuit given a reference mobility ofthe TFT, and the graph having a mark “♦” represents the pixel sizerequired by the 5T pixel driver circuit given a reference mobility ofthe TFT. In FIG. 1, “μ₀” denotes a reference mobility whose value is inthe range 0.1 to 1 cm²/Vs.

For instance, the area of the pixel for the 2-T pixel driver (FIG. 6A)has the area of the switching transistors, the area of the drivetransistor, and the area occupied by interconnects, bias lines, etc. InFIG. 1, the drive current and frame rate are kept constant at 10 μA and50 Hz, respectively, for a 230×230 array. It is clear that there is nosignificant savings in area between the 2-T and 5-T pixel drivers butthe savings are considerable with increasing mobility. This stems mainlyfrom the reduction in the area of the drive transistor where there is atrade-off between TFT and TFT aspect ratio, W/L (Width/Length).

In terms of threshold voltage (V_(T)) uniformity and stability, bothpoly-Si and a-Si:H share the same concerns, although in comparison, thelatter provides far better spatial uniformity but not stability(ΔV_(T)). Thus the inter-pixel variation in the drive current can be aconcern in both cases, although clever circuit design techniques can beemployed to compensate for Δ V_(T) hence improving drive currentuniformity. In terms of long-term reliability, it is not clear withpoly-Si technology. Although there are already products based on a-Si:Htechnology for displays and imaging, the reliability issues associatedwith OLEDs may yet be different.

The fabrication processes associated with a-Si:H technology are standardand adapted from mainstream integrated circuit (IC) technology, but withcapital equipment costs that are much lower. One of the main advantagesof the a-Si:H technology is that it has become a low cost andwell-established technology, while poly-Si has yet to reach the stage ofmanufacturability. The technology also holds great promise forfuturistic applications since deposition of a-Si:H, a-SiN_(x):H, and TFTarrays can be achieved at low temperatures (<120° C.) thus making itamenable to plastic substrates, which is a critical requirement formechanically flexible displays.

To minimize the conduction induced in all TFTs in the pixel by the backelectrode, an alternate TFT structure based on a dual-gate structure isemployed as shown in FIG. 7A. In the dual gate TFT (e.g. FIG. 3), a topgate electrode is added to the TFT structure to prevent the OLEDelectrodes from biasing the a-Si:H channel area (FIG. 2). The voltage onthe top gate can be chosen such so as to minimize the charge induced inthe (parasitic) top channel of the TFT. The objective underlying thechoice of the voltage on the top gate is to minimize parasiticcapacitance in the driver circuits and leakage currents in the TFTs soas to enhance circuit performance. In what follows, the operation of thedual-gate TFT is described.

FIG. 3 illustrates the structure of a dual-gate TFT fabricated for thispurpose, wherein reference numerals S and D denote a source and a drain,respectively. The fabrication steps are the same as of that of a normalinverted staggered TFT structure except that it requires a sixth maskfor patterning the top gate. The length of the TFT may be around 30 μmto provide enough spacing between the source and drain for the top gate.The width may be made large (e.g. 1600 μm) by interconnecting four TFTswith W=400 μm (with four of these TFTs) in parallel to create a sizeableleakage current for measurement. A delay time is inserted in themeasurement of the current to ensure that the measurement has passed thetransient period created by defects in the a-Si:H active layer, whichgive rise to a time-dependent capacitance.

FIG. 4 shows results of static current measurements for four cases:first when the top gate is tied to −10V, second when the top gate isgrounded, third when the top gate is floating, and lastly when the topgate is shorted to the bottom gate. In FIG. 4, V_(tg) represents thebias voltage applied to the top gate of the TFT, and V_(bg) representsthe voltage applied to the bottom gate of the TFT.

With a floating top gate, the characteristics are almost similar to thatof a normal single gate TFT. The leakage current is relatively highparticularly when the top gate is biased with a negative voltage. Thelowest values of leakage current are obtained when the top gate ispegged to either OV or to the voltage of the bottom gate. In particular,with the latter the performance of the TFT in the (forward)sub-threshold regime of operation is significantly improved. Thisenhancement in sub-threshold performance can be explained by the forcedshift of the effective conduction path away from the bottom interface tothe bulk a-Si:H region due to the positive bias on the top gate. This inturn decreases the effect of the trap states at the bottom interface onthe sub-threshold slope of the TFT.

It is noted that although the addition of another metal contact as thetop gate reduces the leakage current of the TFT, it may potentiallydegrade pixel circuit performance by possible parasitic capacitancesintroduced by vertically stacking the OLED pixel. Thus the choice of topgate connection becomes important. For example, if the top gates in thepixel circuit are connected to the bottom gates of the associated TFTs,this gives rise to parasitic capacitances located between the gates andthe cathode, which can lead to undesirable display operation (due to thecharging up of the parasitic capacitance) when the gate driver drivesthe TFT switch as illustrated in FIG. 5. On the other hand, if the topgates are grounded, this results in the parasitic capacitance beinggrounded to yield reliable and stable circuit operation.

The OLED driver circuits considered here are the voltage-programmed 2-Tdriver of FIG. 6A, and the current-programmed ΔV_(T)-compensated 5-Tversion of FIG. 7A. The 5-T driver circuit is a significant variation ofthe previous designs, leading to reduced pixel area, reduced leakage,lower supply voltage, higher linearity (˜30 dB), and larger dynamicrange (40 dB).

Before discussing the operation of the 5-T pixel driver circuit, theoperation of the conventional voltage-driven 2-T pixel driver circuitwill be described. FIG. 6A shows a 2-T pixel circuit including the 2-Tpixel driver circuit, an OLED and a capacitor C_(s). The 2-T pixeldriver includes two TFTs T₁ and T₂. FIG. 6B shows input-output timingchart of the 2-T pixel circuit of FIG. 6A. I_(OLED) represents thecurrent passing through the OLED element and transistor T₂.

Referring to FIGS. 6A and 6B, when the address line is activated byV_(address), the voltage on the data line (V_(data)) starts chargingcapacitor CS and the gate capacitance of the driver transistor T₂.Depending on the voltage on the data line, the capacitor charges up toturn the driver transistor T₂ on, which then starts conducting to drivethe OLED with the appropriate level of current. When the address line isturned off, T₁ is turned off. However, the voltage at the gate of T₂remains since the leakage current of T₁ is trivial in comparison. Hence,the current through the OLED remains unchanged after the turn offprocess. The OLED current changes only the next time around when adifferent voltage is written into the pixel.

FIG. 7A illustrates a 5-T pixel circuit having the 5-T pixel currentdriver circuit for an OLED display, an OLED, and a capacitor C_(s). The5-T pixel current driver circuit has five TFTs T₁-T₅. Unlike the 2-Tpixel driver circuit of FIG. 6A, the data that is written into the 5-Tpixel in this case is a current (I_(data)).

FIG. 7B shows input-output timing diagrams of the 5-T pixel circuit ofFIG. 7A. Referring to FIGS. 7A and 7B, the address line voltageV_(address), and the data line current I_(data) are activated ordeactivated simultaneously. When V_(address) is activated, it forces T₁and T₂ to turn on. T₁ immediately starts conducting but T₂ does notsince T₃ and T₄ are off. Therefore, the voltages at the drain and sourceof T₂ become equal. The current flow through T₁ starts charging the gatecapacitor of transistors T₃ and T₅, like the 2-T driver. The current ofthese transistors starts increasing and consequently T₂ starts toconduct current. Therefore, T₁'s share of I_(data) reduces and T₂'sshare of I_(data) increases. This process continues until the gatecapacitors of T₃ and T₅ charge (via T₁) to a voltage that forces thecurrent of T₃ to be I_(data). At this time, the current of T₅ is zeroand the entire I_(data) goes through T₂ and T₃. At the same time, T₅drives a current through the OLED, which is ideally equal toI_(data)*(W₅/W₃). (W₅/W₃) signifies a current gain where W₅ representschannel width of T₅, and W₃ represents channel width of T₃. Now ifI_(data) and V_(address) are deactivated, T₂ will turn off, but due tothe presence of capacitances in T₃ and T₅, the current of these twodevices cannot be changed easily, since the capacitances keep the biasvoltages constant. This forces T₄ to conduct the same current as that ofT₃, to enable the driver T₅ to drive the same current into the OLED evenwhen the write period is over. Writing a new value into the pixel thenchanges the current driven into the OLED.

The result of transient simulation for the 5-T current driver circuit ofFIG. 7A is shown in FIG. 8. As can be seen, the circuit has a write timeof <70 μs, which is acceptable for most applications. The 5-T currentdriver circuit does not increase the required pixel size significantly(FIG. 1), since the sizes of T₂, T₃, and T₄ are scaled down. This alsoprovides an internal gain (W₅/W₃=8), which reduces the required inputcurrent to <2 μA for 10 μA OLED current.

The transfer characteristics for the 2-T and 5-T driver circuits ofFIGS. 6A and 7A are illustrated in FIGS. 9 and 10, respectively,generated using reliable physically-based TFT models for both forwardand reverse regimes. A much improved linearity (˜30 dB) in the transfercharacteristics (I_(data)/I_(OLED)) is observed for the 5-T drivercircuit due to the geometrically-defined internal pixel gain as comparedto similar designs. In addition, there are two components (OLED and T₅)in the high current path, which in turn decreases the required supplyvoltage and hence improves the dynamic range. According to FIG. 10, agood dynamic range (˜40 dB) is observed for supply voltage of 20V anddrive currents in the range I_(OLED)≦10 μA, which is realistic for highbrightness.

FIG. 11 illustrates variation in the OLED current with the shift inthreshold voltage for the 2-T and 5-T driver circuits of FIGS. 6A and7A.

In FIG. 11, the graph having a mark “▪” represents the OLED current whenusing the 2-T pixel driver circuit, and the graph having a mark “♦”represents the OLED current when using the 5-T pixel driver circuit.

The 5-T current driver circuit compensates for the shift in thresholdvoltage particularly when the shift is smaller than 10% of the supplyvoltage. This is because the 5-T current driver circuit iscurrent-programmed. In contrast, the OLED current in the 2-T drivercircuit changes significantly with a shift in threshold voltage. The 5-Tcurrent driver circuit described here operates at much lower supplyvoltages, has a much larger drive current, and occupies less area.

The pixel architectures are compatible to surface (top) emissive AMOLEDdisplays that enable high on-pixel TFT integration density foruniformity in OLED drive current and high aperture ratio. The 5-T drivercircuit of FIG. 7A provides on-pixel gain, high linearity (−30 dB), andhigh dynamic range (40 dB) at low supply voltages (15-20V) compared tothe similar designs (27V). The results described here illustrate thefeasibility of using a-Si:H for 3-inch mobile monochrome displayapplications on both glass and plastic substrates. With the latter,although the mobility of the TFT is lower, the size of the drivetransistor can be scaled up yet meeting the requirements on pixel areaas depicted in FIG. 1.

As described above, the TFT may be, but not limited to, apolysilicon-based TFT. Polysilicon has higher electron and holemobilities than amorphous silicon. The hole mobilities are large enoughto allow the fabrication of p-channel TFTs.

The advantage of having p-channel TFTs is that bottom emissive OLEDs canbe used along with a p-channel drive TFT to make a good current source.One such circuit is shown in FIG. 12. FIG. 12 illustrates a pixelcircuit having a conventional 2-T polysilicon based pixel current drivercircuit. The 2-T polysilicon based pixel current driver circuit has ap-channel drive TFT. In FIG. 12, T₁ and T₂ are p-channel TFTs.

In FIG. 12, the source of the p-type drive TFT is connected toV_(supply). Therefore, Vgs, gate-to-source voltage, and hence the drivecurrent of the p-type TFT is independent of OLED characteristics. Inother words, the driver shown in FIG. 12 performs as a good currentsource. Hence, bottom emissive OLEDs are suitable for use with p-channeldrive TFTs, and top emissive OLEDs are suitable for use with n-channelTFTs.

The trade-off with using polysilicon is that the process of makingpolysilicon TFTs requires much higher temperatures than that ofamorphous silicon. This high temperature-processing requirement greatlyincreases the cost, and is not amenable to plastic substrates. Moreover,polysilicon technology is not as mature and widely available asamorphous silicon. In contrast, amorphous silicon is a well-establishedtechnology currently used in liquid crystal displays (LCDs). It is dueto these reasons that amorphous silicon combined with top emissive OLEDbased circuit designs is most promising for AMOLED displays.

Compared to polysilicon TFTs, amorphous silicon TFTs are n-type and thusare more suitable for top emission circuits as shown in FIG. 2, anddoesn't preclude their use in full colour bottom emission circuitseither. However, amorphous silicon TFTs have inherent stability problemsdue to the material structure. In amorphous silicon circuit design, thebiggest hurdle is the increase in threshold voltage V_(th) afterprolonged gate bias. This shift is particularly evident in the drive TFTof an OLED display pixel. This drive TFT is always in the ‘ON’ state, inwhich there is a positive voltage at its gate. As a result, its V_(th)increases and the drive current decreases based on the current-voltageequation below:

Ids=(μC _(ox) W/2L)(V _(gs) −V _(th))²(in Saturation region)

where Ids represents drain to source current; μ represents mobility;C_(ox) represents gate capacitance; W represents channel width; Lrepresents channel length; V_(gs) represents gate to source voltage; andV_(th) represents threshold voltage.

In the display, this would mean that the brightness of the OLED woulddecrease over time, which is unacceptable. Hence, the 2-T drivercircuits as described above are not practical for OLED displays, as theydo not compensate for any increase in V_(th).

By contrast, the current mirror based pixel current driver circuitillustrated in FIG. 7A automatically compensates for shifts in theV_(th) of the drive TFT in a pixel.

FIGS. 13-17 illustrate pixel circuits having pixel current drivercircuits in accordance with further embodiments of the presentinvention. Each of the pixel circuits shown in FIGS. 13-16 includes a4-T pixel current driver circuit, an OLED and a capacitor C_(s). Thepixel circuit shown in FIG. 17 includes a pixel current driver circuit,OLEDs, and capacitors C_(s). While the pixel current driver circuits ofFIGS. 13-16 are presented for a monochrome OLED display, the pixelcurrent driver circuits of FIGS. 13-16 are, however, applicable to afill color display. The pixel current driver circuit of FIG. 17 isprovided for a full colour, OLED display.

The pixel driver circuits of FIGS. 13-17 are current mirror based pixeldriver circuits. All these circuits illustrated in FIGS. 13-17 havemechanisms that automatically compensate for the V_(th) shift of a driveTFT.

The pixel current driver circuit of FIG. 13 is a modification of the 5-Tpixel driver circuit of FIG. 7A. The 4-T pixel current driver circuit ofFIG. 13 has four TFTs, T₁-T₄. The 4-T pixel current driver circuit ofFIG. 13 compensates for the shift of V_(th) of T₄. The 4-T pixel currentdriver circuit of FIG. 13 occupies a smaller area than that of the 5-Tpixel current driver circuit, and provides a higher dynamic range. Thehigher dynamic range allows for a larger signal swing at the input,which means that the OLED brightness can be adjusted over a largerrange.

The 4-T pixel current driver circuit of FIG. 14 has four TFTs, T₁-T₄,and has a lower discharge time. The 4-T pixel current driver circuit ofFIG. 14 compensates for the shift of V_(th) of T₄. The advantage of thiscircuit is that the discharge time of the capacitor C_(s) issubstantially reduced. This is because the discharge path has two TFTs(as compared to three TFTs in the circuit of FIG. 13). The charging timeremains the same. The other advantage is that there is an additionalgain provided by this circuit because T₃ and T₄ do not have the samesource voltages. However, this gain is non-linear and may not bedesirable in some cases.

The 4-T pixel current driver circuit of FIG. 15 has four TFTs, T₁-T₄.The 4-T pixel current driver circuit of FIG. 15 compensates for theshift of V_(th) of T₄. This circuit does not have the non-linear gainpresent in the pixel driver circuit of FIG. 14, since the sourceterminals of T₃ and T₄ are at the same voltage. It still maintains thelower capacitance discharge time, along with the other features of thecircuit of FIG. 9.

The 4-T pixel current driver circuit of FIG. 16 has four TFTs, T₁-T₄.The 4-T pixel current driver circuit of FIG. 16 compensates for theshift of V_(th) of T₄. This circuit forms the building block for the3-colour RGB circuit shown in FIG. 17. It also has a low capacitancedischarge time and high dynamic range.

The full colour circuit shown in FIG. 17 minimizes the area required byan RGB pixel on a display, while maintaining the desirable features likethreshold voltage shift compensation, in-pixel current gain, lowcapacitance discharge time, and high dynamic range. In FIG. 17,V_(blue), V_(Green), V_(Red) represent control signals for programmingthe blue, green, and red pixels, respectively. The pixel current drivercircuit of FIG. 17 compensates for the shift of V_(th) of T₆.

The circuits described above may be fabricated using normal invertedstaggered TFT structures. The length and width of the thin filmtransistors may change depending on the maximum drive current requiredby the circuit and the fabrication technology used.

The pixel current driver circuits of FIGS. 7 and 13-17 use n-typeamorphous silicon TFTs. With the above structure on the a-Si:H currentdriver according to the embodiments of the present invention, the chargeinduced in the top channel of the TFT is minimized, and the leakagecurrents in the TFT is minimized so as to enhance circuit performance.

However, polysilicon technology may be applied to the pixel currentdriver circuits using p-type or n-type TFTs. These circuits, when madein polysilicon, can compensate for the spatial non-uniformity of thethreshold voltage. The p-type circuits are conjugates of theabove-mentioned circuits and are suitable for the bottom emissivepixels.

In FIGS. 6A, 7A, and 12-17, the TFT having dual gates is shown, wherethe dual gate includes a top gate and a bottom gate. The top gate may begrounded (for example, in FIGS. 6A, 7A and 12-17), or electrically tiedto a bottom gate (FIG. 18).

The dual-gate TFTs are used in the above-mentioned circuits to enablevertical integration of the OLED layers with minimum parasitic effects.However, the above-mentioned circuits compensate for the V_(th) shiftwhen the circuits comprise single-gate TFTs.

FIGS. 19-24 illustrate pixel current driver circuits having single-gateTFTs. FIGS. 19-24 correspond to FIGS. 7A and 13-17, respectively.

For example, the pixel current driver circuit of FIG. 19 containssingle-gate TFTs having a switch TFT T₁, a feedback TFT T₂, a referenceTFT T₃, a diode-use TFT T₄, and a drive TFT T₅. The pixel current drivercircuit of FIG. 20 contains single-gate TFTs having a switch TFT T₁, afeedback TFT T₂, a reference TFT T₃, and a drive TFT T₄. The pixelcurrent driver circuit of FIG. 22 contains single-gate TFTs having afeedback TFT T₁, a switch TFT T₂, a reference TFT T₃, and a drive TFTT₄. The pixel current driver circuit of FIG. 24 contains single-gateTFTs having switch TFTs T₁, T₃, T₄, T₅, a feedback TFT T₂, and drive TFTT₆, T₇, T₈.

The driving scheme and operation of the pixel driver circuits of FIGS.19-24 are same as those of FIGS. 7A and 13-17. The major differencebetween the pixel current driver circuit having dual-gate TFTs and thepixel current driver circuit having single-gate TFTs is that the pixelcurrent driver circuit having the dual-gate TFTs utilize a better TFTdesign which minimizes the leakage currents in the TFTs, thus enhancingcircuit performance. However, the single-gate TFTs are the standard andpreferred design in industry.

In FIGS. 19-24, n-type TFTs are shown. However, the pixel current drivercircuits having single-gate TFTs may include p-type TFTs. Pixel drivercircuits with p-type TFTs are shown in FIG. 25-30, where the circuitsfor FIGS. 25-30 are analogous to those of FIG. 19-24, respectively.

With regard to the current driver circuits of FIGS. 19-30 the OLEDs canbe either non-inverted or inverted. The four possible cases arepresented in Table 1.

TABLE 1 Possible OLED connections. Bottom Emission Top Emission OLEDReduced aperture ratio Large aperture ratio connected at Regular OLED -Regular OLED - source of transparent anode transparent cathode. driveTFT OLED current depends on OLED current depends OLED voltage which onOLED voltage which changes with aging - changes with aging - undesirablelocation undesirable location Safeguards against Safeguards againstsmall small variation in drive variation in drive current by sourcecurrent by source degeneration degeneration OLED Reduced aperture ratioLarge aperture ratio connected at Inverted OLED - Inverted OLED - drainof drive transparent cathode transparent anode TFT OLED currentindependent OLED current of OLED voltage independent of OLED voltage

The present invention has been described with regard to one or moreembodiments. However, it will be apparent to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as defined in the claims.

1-35. (canceled)
 36. A pixel driver circuit comprising: an address line;a data line; a switch thin film transistor, a first node of the switchtransistor being connected to the data line and a gate of the switchtransistor being connected to the address line; a feedback thin filmtransistor, a gate of the feedback transistor being connected to theaddress line and a second node of the feedback transistor beingconnected to a potential; a reference thin film transistor, a first nodeof the reference transistor being connected to a second node of theswitch transistor, a gate of the reference transistor being connected tothe second node of the switch transistor, and a second node of thereference transistor being connected to a first node of the feedbacktransistor; and a drive thin film transistor, a gate of the drivetransistor being connected to the gate of the reference transistor. 37.The pixel driver circuit according to claim 36, wherein at least one ofthe thin film transistors is an amorphous silicon based thin filmtransistor.
 38. The pixel driver circuit according to claim 36, whereinat least one of the thin film transistor is a polycrystalline siliconbased thin film transistor.
 39. The pixel driver circuit according toclaim 36, wherein at least one of the thin film transistors is a n-typethin film transistor.
 40. The pixel driver circuit according to claim36, wherein at least one of the thin film transistors is a p-type thinfilm transistor.
 41. The pixel driver circuit according to claim 36,wherein the thin film transistors each comprise a second gate.
 42. Thepixel driver circuit according to claim 36, wherein the first node ofthe reference transistor is a drain node, and wherein the second node ofthe reference transistor is a source node.
 43. The pixel driver circuitaccording to claim 36, wherein the second node of the feedbacktransistor is connected to a ground potential.
 44. The pixel drivercircuit according to claim 36, wherein the second node of the feedbacktransistor is connected to a voltage supply.
 45. The pixel drivercircuit according to claim 36, comprising a capacitor connected to thegate of the drive transistor and a ground potential.
 46. The pixeldriver circuit according to claim 36, comprising a capacitor connectedto the gate of the drive transistor and a voltage supply.
 47. A pixelcircuit comprising: a pixel driver circuit according to claim 36; and anorganic light emitting diode, the one of a first node and a second nodeof the drive transistor being connected to the organic light emittingdiode.
 48. The pixel circuit according to claim 47, wherein the one ofthe first node and the second node of the drive transistor is connectedto the organic light emitting diode, and wherein the other node isconnected to a ground potential.
 49. The pixel circuit according toclaim 47, wherein the one of the first node and the second node of thedrive transistor is connected to the organic light emitting diode, andwherein the other node is connected to a voltage supply.
 50. The pixelcircuit according to claim 47, wherein the one of the first node and thesecond node of the drive transistor is a drain, and wherein the other isa source.
 51. The pixel circuit according to claim 47, comprising acapacitor connected between the gate of the drive transistor and aground potential.
 52. The pixel circuit according to claim 47,comprising a capacitor connected between the gate of the drivetransistor and a voltage supply.
 53. The pixel circuit according toclaim 47, wherein the pixel circuit is arranged for a monochromedisplay.
 54. The pixel circuit according to claim 47, wherein the pixelcircuit is arranged for a full color display.